FalconSat-1 is the first of a series of small satellites, designed, built and operated by cadets at the US Air Force Academy, Colorado Springs, CO. The overall science objective of the mission is to determine the effects of S/C charging in the LEO environment. Other objectives are to demonstrate/validate the S/C design and to provide a hands-on experience for all students involved.

Spacecraft:

The S/C structure is nearly cubical in shape with dimensions: 44.5 cm x 44.5 cm x 43.2 cm. The solar arrays (GaAs cells of 19% efficiency) are body-mounted providing 24 W of power, in addition there is a battery with 10 NiCd cells (4.4 Ah). The bus design features a stack assembly into which the subsystems are mounted. The mass of the microsatellite is 52 kg. 1)2)3)4)

Figure 1: Illustration of FalconSat-1 (image credit: USAFA)

The S/C is spin-stabilized at about 10 rpm pointing toward nadir (a minimal pointing of ±25º is required in the direction of S/C velocity vector). The ADACS (Attitude Determination and Control Subsystem) measures attitude with a three-axis magnetometer; a torque rod functions as an actuator, damping is provided by hysteresis rods. The launch vehicle imparts an initial spin on S/C separation. In addition, nylon solar radiation paddles are used to maintain the initial spin rate. By using magnetometer data contained in the telemetry stream, the satellite can be commanded to operate the torque rod as necessary to align itself with the Earth's magnetic field lines. Due to the shape of the Earth's field lines, the spacecraft executes a “flip,” turning 180º over at each crossing of the equator, when the torque rod is operating on a 100% duty cycle.

The C&DHS (Command and Data Handling Subsystem) consists of three main components, the flight computer (FC) and two embedded controllers. The flight computer is a NEC V53 space-qualified flight computer with 1 MB of EDAC program memory and a 4 MB RAM disk. A real-time multitasking operating system is used. The AX.25 protocol stack is implemented.

Figure 2: Block diagram of the C&DH subsystem (image credit: USAFA)

Launch: FalconSat-1 was launched on Jan. 27, 2000 (UTC) with a converted Minuteman-II missile (Minotaur) from VAFB, CA.. The Minotaur of OSC is a four-stage vehicle with the first and second stages being Minuteman-II stages; the two upper stages come from OSC's Pegasus launcher.

FalconSat-1 was part of the JAWSAT multi-payload adapter and launched with the following satellites: ASUSat-1, OPAL (PicoSat, Artemis, StenSat), OSCE, MASat, and JAWSAT.

RF communications are provided in UHF (downlink) at 400.475 MHz and in VHF (uplink) at 148.030 MHz. The data rate for transmit and receive is 9.6 kbit/s. The signal modulation scheme employs GMSK (Gaussian Minimum Shift Keying). - The ground station for FalconSat-1 is located at USAFA providing monitoring and control of the spacecraft.

Mission status:

• The FalconSat-1 spacecraft failed on-orbit soon after a successful deployment, apparently due to a power failure. In the following weeks, cadets working in the Academy ground station struggled to bring the satellite totally on-line. Initial communication contacts with the satellite went well, but during the subsequent period, it became apparent that the spacecraft's power system was not functioning correctly to properly charge the batteries during daylight. Unfortunately, after about 1 month, the mission was satellite the mission was terminated (Ref. 4).

A USAF press statement of June 2002 said: “While FalconSat-1 was a technical failure, it was a resounding academic success. Cadets participated in all phases of the mission from conceptual design though assembly, integration, testing, launch and on-orbit operations.”

CHAWS-LD was developed by the Physics Department at the Academy and supported by DoD. The objective was to measure electric charge characteristics of the S/C. Voltage and current sensors, made of sheets of stainless steel (and an electric circuit board stacked together with aluminum and Teflon spacers), are installed on the four sides of the S/C.

The sensors are made of sheets of stainless steel and an electric circuit board material stacked together with aluminum and Teflon spacers. Each sensor requires a 10 cm x 10 cm opening on the inside surface of the spacecraft structure and a 9 cm x 9 cm opening on the outside of the structure. One voltage and one current sensor are mounted in the center of each side of the spacecraft. The surface of the voltage sensors, which is exposed to the plasma, is the metalized surface of a circuit board. The exposed surface of the current sensors is an electro-formed stainless steel mesh. The detector electronics are sealed from the space environment.

Figure 3: Layout of the CHAWS-LD sensors (image credit: USAFA)

As FalconSat moves through the space plasma, it creates a wake region behind it, in which primarily electrons accumulate. Hence, the electrically isolated sections of the voltage sensors on the wake side are negatively charged. The current sensors reject electrons and collect ions from the space plasma, providing a current that is correlated to the ambient plasma density. The relative amount of current collected by each sensor provides in addition information about the S/C attitude relative to the plasma flow. Data measurements can be made at 5 or at 10 Hz. A series of measurements is taken over the course of one or more orbits. - One voltage and one current sensor are center-mounted at side of the S/C. The surface of the voltage sensors, exposed to the plasma, is the metalized surface of the circuit board. The exposed surface of the current sensors is an electro-formed stainless steel mesh. The data collected from each sensor are stored in the memory of the flight computer. They can be transmitted to the ground during station passes.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.